A synchronous-rectification-type linear solenoid driving system according to the related art will be described with reference to FIGS. 9 and 10. FIG. 9 is a circuit diagram illustrating the synchronous-rectification-type linear solenoid driving system. FIG. 9 illustrates a synchronous rectification circuit that is a switching circuit for driving a linear solenoid which is a load. In the synchronous rectification circuit illustrated in FIG. 9, a vertical n-channel MOSFET 52 which is a high-side switch and a horizontal n-channel MOSFET 53 which is a low-side switch are connected in series to each other between a power supply terminal 57 and a ground terminal 58.
In FIG. 9, reference numeral 54 indicates a body diode (parasitic diode) of the vertical MOSFET 52 and reference numeral 55 indicates a body diode (parasitic diode) of the horizontal MOSFET 53. A linear solenoid 56 is connected as a load to an output terminal 59 which is a connection portion between a source of the vertical MOSFET 52 and a drain of the horizontal MOSFET 53. A control circuit 51 outputs signals to a gate terminal 60 and a gate terminal 61 to control the operation of the vertical MOSFET 52 and the horizontal MOSFET 53. In FIG. 9, reference numeral 62 indicates a ground terminal.
FIG. 10 is a timing chart illustrating the output signals from the control circuit 51. In FIG. 10, the upper waveform is the timing chart of the vertical MOSFET 52 and the lower waveform is the timing chart of the horizontal MOSFET 53.
Next, a synchronous rectification operation when the control circuit 51 outputs the signals illustrated in FIG. 10 in the synchronous-rectification-type linear solenoid driving system illustrated in FIG. 9 will be described. For a period Ton1, the vertical MOSFET 52 is turned on and a current is supplied from the power supply terminal 57 to the linear solenoid 56. When the period Ton1 ends, the vertical MOSFET 52 is turned off and the current which flows to the linear solenoid 56 starts to be reduced. At that time, electromotive force is generated in the linear solenoid 56 in order to keep the current flowing and the potential of the output terminal 59 is lower than the ground potential.
For a period Ton2, the horizontal MOSFET 53 is turned on and a current flows from the ground terminal 58 to the output terminal 59 to supply a return current IL to the linear solenoid 56. The synchronous rectification circuit makes the return current IL flow to the horizontal MOSFET 53 with low resistance to suppress loss. When the horizontal MOSFET 53 is turned on and a channel is opened, the return current IL flows from the source to the drain of the horizontal MOSFET 53 through the channel.
When the vertical MOSFET 52 and the horizontal MOSFET 53 are turned on at the same time, there is a concern that an overcurrent will flow from the power supply terminal 57 to the ground terminal 58 and a defect will occur in the system. Therefore, in the synchronous-rectification-type linear solenoid driving system, during the synchronous rectification operation, a dead time period Td is set between the periods Ton1 and Ton2 to prevent the high-side vertical MOSFET 52 and the low-side horizontal MOSFET 53 from being turned on at the same time. For the period Td, the vertical MOSFET 52 and the horizontal MOSFET 53 are not turned on. Therefore, the return current IL to the linear solenoid 56 is supplied from the ground terminal 58 through the body diode 55 of the horizontal MOSFET 53. The synchronous rectification circuit performs PWM control for changing the duration of the periods Ton1 and Ton2 to change the amount of current supplied, thereby controlling the operation of the linear solenoid 56.
When the high-side switch and the low-side switch of the synchronous rectification circuit are formed by MOSFETs, the following structures are considered: a structure in which two vertical MOSFETs are used; a structure in which two horizontal MOSFETs are used; and a structure in which one vertical MOSFET and one horizontal MOSFET are used.
When two vertical MOSFETs are used, it is necessary to form the two vertical MOSFETs with separate chips in order to connect the vertical MOSFETs in series. In addition, when two vertical MOSFETs are used, in general, a control circuit which is configured by a horizontal MOSFET is formed in a separate chip or it is formed in the same chip together with one of the MOSFET chips.
When two horizontal MOSFETs are used or when one vertical MOSFET and one horizontal MOSFET are used, the MOSFETs can be formed with separate chips, similarly to when two vertical MOSFETs are used, or a so-called power IC can be used in which the MOSFETs and the control circuit are formed in the same chip.
In many cases, the synchronous rectification circuit is used in a DC-DC converter system. In order to reduce the size and costs of the system, a semiconductor device has been proposed in which the chips are accommodated in the same package. For example, the following semiconductor devices have been proposed: a semiconductor device for synchronous rectification which has a one-chip structure using two horizontal MOSFETs; and a semiconductor device for synchronous rectification which has a two-chip structure using a vertical MOSFET and a horizontal MOSFET (for example, see the following Patent Document 1).
In addition, for example, a power IC has been proposed in which a vertical n-channel MOSFET is formed on the high side and a horizontal n-channel MOSFET is formed on the low side, using a portion SOI (for example, see the following Patent Document 2). For example, a power IC has been proposed in which a horizontal n-channel MOSFET is formed on the high side and a vertical n-channel MOSFET is formed on the low side (for example, see the following Patent Document 3). For example, a semiconductor device has been proposed in which a p-channel MOSFET is shunted to suppress the operation of a parasitic diode (for example, see the following Patent Document 4).